Surgical motor systems of the generic kind are known in numerous variants, in particular together with tools in the form of drilling and milling machines or saws. They are operated by the motor control unit generating control signals for the electric motor, to operate it with a specific rotational speed which can be set by the drive control device. Depending on the type of the electric motor, rotational speeds of up to 80000 revolutions per minute may be achieved. A motor system of this type is especially cost-effective and low-maintenance if the electric motor is a brushless DC motor which has at least two motor windings apart from the rotor.
Drive control devices for surgical motor systems of the generic kind are known, for example, from WO 2006/012991 A1 or DE 10 2009 018 143 A1. With these known drive control devices, the rotational speed of the surgical tool concerned is controlled in a closed loop, wherein e.g. a pulse width modulation (PWM) or a space vector pulse width modulation (SVPWM) is used. Compared to the conventional pulse width modulation, the SVPWM method has the advantage that all motor windings can be energized at the same time, so that a smooth and jerk-free operation of the electric motor is possible also with particularly low rotational speeds.
From EP 2 324 779 A1 a surgical motor system is known intended for various medical applications and comprising a drive control unit including an electronic circuit serving to control the rotational speed of the motor system in an open and/or closed loop. The way of controlling the rotational speed in an open and/or closed loop changes depending on the tool holder which is attached to the surgical motor system.
Apart from their smooth operation, these known closed-loop control methods are distinguished above all by the fact that the respective rotational speed can be readjusted very fast. However, it has turned out that e.g. in the event of extreme load changes a very quick readjustment of the rotational speed does not only bring advantages, as can be seen from the following comparison:
If e.g. an uncontrolled motor operated with compressed air experiences a short-term drop of the rotational speed during driving a milling cutter due to it being possibly caught, this will result in the motor speed rising again after the milling cutter has come loose. An additional temperature rise will not be produced here.
On the other hand, if such a milling cutter is driven by an electric motor controlled in a closed loop, the motor control unit performs the readjustment in such a situation within very short time to counteract the sharp drop of the rotational speed. This results in problems in particular if the tool gets caught several times in succession and the rotational speed has to be readjusted again and again. If the current profile is examined in more detail in such a situation, additional energization impulses will be measured (I2×R). As a consequence, these very quick, dynamic closed-loop control processes are accompanied by additional losses. Due to the continuous readjusting and the associated current peaks, the electric motor produces a larger amount of heat and there is the risk that it will overheat.
There are many applications (e.g. in craniotomy) in which such a quick readjusting is desired or even required, where the depicted disadvantage of the additional losses may be simply accepted due to the comparably short activation times in craniotomy. However, there are applications such as the time-consuming process of milling off a bone segment in which the focus is not so much on the quick readjustment but rather on the lowest possible thermal losses.
If the above-mentioned drive control devices known e.g. from WO 2006/012991 A1 or DE 10 2009 018 143 A1 are used in a process of milling off a bone segment, there is the risk that the electric motor will be overheated or possibly even damaged. Accordingly, the field of use of the previously known drive control devices for surgical instruments or apparatus is limited to specific purposes for reasons of safety. This is why different surgical motor systems or those comprising respectively suitable drive motors are used hitherto depending on the field of application. This impedes the handling and increases costs as well. It would be desirable, however, to use one motor system for a wide variety of medical applications.